1. Field of the Invention
This invention relates to frequency synthesizers and more particularly to the field of frequency synthesizers for producing noise with prederetermined amplitudes within predetermined frequency bands. The invention frequency synthesizer is particularly adapted for use in synthesizing the acoustical signatures, or sounds, of objects moving in an undersea environment, such as those sounds made by ambient ocean noise, currents propulsion, and machinery noise.
2. Description of the Prior Art
There are no known techniques for the direct synthesis of band limited noise of arbitary shape. A common method for noise generation is to pass a wide band noise source through a predetermined network of filters. Each filter then limits the noise to its respective band. The outputs of each respective filter are then scaled and summed to construct a predetermined composite response. This standard filter approach has been implemented with both analog and digital circuits. The noise spectrum within each predetermined band can be changed by calculating a new set of filter coefficients and/or changing the gain scalers for each respective band's filter.
The prior art approach to synthesizing noise with controlled bands is costly to implement. Requirements for a frequency synthesizer of this type requires design of many filter elements to obtained the required spectrum shape characteristics. In addition to the design and cost of the the filters, additional time and hardware must be expended to implement any required spectrum shape changes. For example, a prior art frequency synthesizer of the above referenced type, required to provide a seven octave band of noise with one-third octave segments, would require 21 paralleled multiple stage narrow band filters. This represents a significant amount of hardware and cost and demonstrates a substantial lack of flexibility.